Peer-to-peer (P2P) communication systems allow computational entities (peers) to establish software connections (virtual channels) between one another. P2P systems therefore allow peers to communicate or share computational tasks and resources without the explicit need for centralised control. P2P can operate in a generalised network having one or more servers: a peer may provide information (publish) to at least one service on the network and/or register (subscribe) with services on that network to receive information published by another peer.
Messaging systems that benefit from the provision of centralised control are also known. Here, all messages are directed from publishers to subscribers, via a central locus where some computation (mediation) is performed on the messages. New messages (digests, for example) are generated from the input messages and sent to appropriate subscribers.
In prior art centralised mediation systems, all message traffic is transmitted through a central network point (locus), where the mediation service resides. Viewed in terms of logical elements, such systems are constructed as a star-shaped architectural model with a central point of control, where mediation tasks are executed. This model is shown in FIG. 1A: each source (publisher) and sink (subscriber) of information has a line of communication that connects to the central mediation hub. In many cases, the sources and sinks represent the same entities operating in different modes, and may not be architecturally distinguishable.
The problems associated with such an architecture are well known. They are prone to suffer from a lack of bandwidth at the point of mediation. Even though the logical star shape may be superimposed upon a physical network that is highly connected, the essential flow of all information through a central point causes an inherent throughput bottleneck, based upon the bandwidth available between this point and the network (see FIG. 1B). Although advances in networking technologies mean that bandwidth availability continues to improve, increasing bandwidth has an inherent financial cost, and in certain scenarios can cause a real limitation to the throughput of the overall system. This limitation is manifested as a restriction on either the maximum number of users, or the rate at which each user is able to send and receive information.
Indeed there are many examples of systems where neither P2P architectures nor centralised mediation architectures are wholly satisfactory. Often some logical process is required to act over the sum of messages broadcast within the messaging system. Examples of classes of systems where neither architecture is completely suitable include: a trading system where potential buyers and sellers advertise to each other, mediation is required to ensure a transactional matching of requirements; a mediated news or publishing system where a central authority acts as the editorial control, before information is disseminated; a system which is not actively controlled but which requires an ordered log of information flow to be maintained in a central repository; a conversation service which allows a recent context to be presented to a user joining an ongoing conversation; distributing cryptographic keys (the so-called key distribution problem); systems for finding the location of data (state) and services on a distributed network; and systems for locating and communicating with mobile users.
All the examples above have in common a requirement both for peer-to-peer communication, and for a degree of centralised mediation of the flow of information when the communication is viewed as a whole.
It is therefore an object of the invention to obviate or at least mitigate the aforementioned problems.